Heat pipe and method for making the same

ABSTRACT

A heat pipe includes a casing ( 100 ) receiving working fluid therein, and a wick structure ( 200 ) formed at an inner wall of the casing. The casing includes an evaporating section ( 120 ) and a condensing section ( 140 ). The wick structure at the evaporating section has a thickness larger than that of the wick structure at the condensing section. A method for manufacturing the wick structure includes the following steps: providing a hollow casing ( 10 ) arranged aslant to horizon; filling a slurry ( 40 ) of powders into the casing; rotating the casing with the slurry and heating the slurry to form a green layer ( 60 ) held against an inner wall of the casing; and sintering the green layer in the casing to form the wick structure.

FIELD OF THE INVENTION

The present invention relates generally to an apparatus for transfer or dissipation of heat from heat-generating components such as electronic components, and more particularly to a method of manufacturing a wick structure for a heat pipe.

DESCRIPTION OF RELATED ART

Heat pipes have excellent heat transfer performance due to their low thermal resistance, and therefore are an effective means for transfer or dissipation of heat from heat sources. Currently, heat pipes are widely used for removing heat from heat-generating components such as central processing units (CPUs) of computers. A heat pipe is usually a vacuum casing containing therein a working fluid, which is employed to carry, under phase transitions between liquid state and vapor state, thermal energy from one section of the heat pipe (typically referring to as the “evaporating section”) to another section thereof (typically referring to as the “condensing section”). The casing is made of high thermally conductive material such as copper or aluminum. Preferably, a wick structure is provided inside the heat pipe, lining an inner wall of the casing, for drawing the working fluid back to the evaporating section after it is condensed at the condensing section.

The wick structure currently available for heat pipes includes fine grooves integrally formed at the inner wall of the casing, screen mesh or bundles of fiber inserted into the casing and held against the inner wall thereof, or sintered powders combined to the inner wall by sintering process. Among these wicks, the sintered powder wick is preferred to the other wicks with respect to heat transfer ability and ability against gravity. Currently, a conventional method for making a sintered powder wick includes inserting a column-shaped mandrel at a central portion of a hollow casing which has a closed end and an open end. Powders are filled into the casing to construct the wick. The mandrel functions to hold the filled powders against an inner wall of the casing. Then, the casing with the powders is sintered at high temperature for a specified time period to cause the powders to diffusion bond together to form the wick. As a result, the wick structure has an even thickness along an axial direction of the heat pipe.

Since the primary function of a wick is to draw condensed liquid back to the evaporating section of a heat pipe under the capillary pressure developed by the wick, the capillary pressure is an important parameter affecting the performance of the wick. Since it is well recognized that the capillary pressure of a wick increases due to an increase in amount of pores of the wick. As the area of the wick structure being limited to the size of the casing of the heat pipe, the way to enhance the amount of pores is to increase the thickness of the wick structure. However, the thermal resistance of the wick structure increases with the increasing of the thickness of the wick structure. The heat carried by the vapor is difficult to dissipate to the casing and then to ambient when the wick structure has a large thickness. Then the vapor can not condense to liquid rapidly. As a result, a heat pipe with a wick having evenly thickness often suffers dry-out problem at the evaporating section as the condensed liquid cannot be timely sent back to the evaporating section of the heat pipe.

Therefore, there is a need for a heat pipe with a sintered powder wick which can provide simultaneously a relatively large capillary force and a relatively low thermal resistance so as to effectively and timely bring the condensed liquid back from its condensing section to its evaporating section and thereby to avoid the undesirable dry-out problem at the evaporating section.

SUMMARY OF THE INVENTION

A heat pipe in accordance with a preferred embodiment of the present invention includes a casing receiving working fluid therein, and a wick structure formed at an inner wall of the casing. The casing includes an evaporating section and a condensing section. The wick structure at the evaporating section has a thickness larger than that of the wick structure at the condensing section of the heat pipe.

The present invention in another aspect, relates to a method for manufacturing the sintered powder wick structure of the heat pipe. The preferred method includes steps of: providing a hollow casing arranged aslant to horizon; filling a slurry of powders into the casing; rotating the casing with the slurry to form a slurry layer held against an inner wall of the casing; and sintering the slurry layer in the casing to form the wick structure.

Other advantages and novel features of the present invention will be drawn from the following detailed description of the preferred embodiments of the present invention with attached drawings, in which:

BRIEF DESCRIPTION OF THE DRAWINGS

Many aspects of the present heat pipe the method can be better understood with reference to the following drawings. The components in the drawings are not necessarily drawn to scale, the emphasis instead being placed upon clearly illustrating the principles of the present heat pipe the method. Moreover, in the drawings, like reference numerals designate corresponding parts throughout the several views.

FIG. 1 is a longitudinal cross-sectional view of a heat pipe in accordance with a preferred embodiment of the present invention;

FIG. 2 shows a flow chart of a preferred method in manufacturing the heat pipe of FIG. 1;

FIGS. 3-5 are schematic diagrams of one example of the method, showing different stages in forming the wick structure by using the method of FIG. 2; and

FIGS. 6-8 are schematic diagrams of another example of the method, showing different stages in forming the wick structure by using the method of FIG. 2.

DETAILED DESCRIPTION OF THE INVENTION

FIG. 1 illustrates a heat pipe in accordance with a preferred embodiment of the present invention. The heat pipe is vacuumed and includes a casing 100 and a sintered powder wick structure 200 arranged against an inner wall of the casing 100. The heat pipe is divided into an evaporating section 120, an adiabatic section 130 and a condensing section 140 along an axial direction of the heat pipe. The adiabatic section 130 is located between the evaporating and condensing sections 120, 140.

The casing 100 is made of high thermally conductive material such as copper or aluminum. Although the casing 100 illustrated is in a round shape, it should be recognized that other shapes, such as polygon, rectangle, or triangle, may also be suitable.

The wick structure 200 is saturated with a working fluid (not shown), which acts as a heat carrier when undergoing phase transitions between liquid state and vaporous state. The wick structure 200 is a porous structure and is formed by sintering process, in which small-sized powders are sintered together under high temperature. The wick structure 200 is in the form of an uneven structure. A thickness of the wick structure 200 gradually decreases along an axial direction from the evaporating section 120 to the condensing section 140 of the heat pipe. The wick structure 200 has the largest thickness at the evaporating section 120, whilst has the smallest thickness at the condensing section 140 of the heat pipe. In other words, the thickness of the wick structure 200 at the evaporating section 120 is larger than that at the condensing section 140.

During operation, the evaporating section 120 of the heat pipe is maintained in thermal contact with a heat-generating component (not shown) such as a CPU. The working fluid contained at the evaporating section 120 absorbs heat generated by the heat-generating component and then turns into vapor. Due to the difference of vapor pressure between the two sections 120, 140 of the heat pipe, the generated vapor moves towards and carries the heat simultaneously to the condensing section 140 where the vapor is condensed into liquid after releasing the heat into ambient environment by, for example, fins (not shown) thermally contacting the condensing section 140. Due to the difference of capillary pressure developed by the wick structure 200 between the two sections 120, 140, the condensed liquid is then drawn back by the wick structure 200 to the evaporating section 120 where it is again available for evaporation. For the wick structure 200 corresponding to the evaporating section 120 of the heat pipe having the largest thickness, the evaporating portion of the wick structure 200 has a relatively larger amount of pores than other portions thereof. Therefore the evaporating portion has a relatively larger capillary force to draw back the condensed working fluid to the evaporating section 120. At the same time, the wick structure 200 corresponding to the condensing section 140 of the heat pipe has the smallest thickness. Thus the condensing portion of the wick structure 200 has a relatively smaller thermal resistance. The vapor is easily to dissipate the heat absorbed from the heat-generating component to the casing 100 of the heat pipe, and then transfers to the ambient environment. Thus the vapor is capable of condensing to liquid and then flowing back to the evaporating section 120 timely. Therefore, dry-out of the heat pipe is avoided. As a result, the heat of the heat-generating component can be dissipated appropriately.

In the present invention, a method as shown in FIG. 2 is proposed to construct the wick structure 200 of the heat pipe. Also refer to FIGS. 3-5. The method includes a first step of providing a hollow casing 10. The casing 10 is cylinder-shaped with two open ends 16, 18 (i.e., first and second open ends 16, 18). A hole 160, 180 is defined in each of the open ends 16, 18. Each end 16, 18 has an inner diameter and an outer diameter smaller than that of the other portion of the casing 10. The casing 10 is set on a centrifugal forming machine 30. The machine 30 has a top surface 32 for mounting the casing 10 thereon. The top surface 32 is slanted. Thus, the casing 10 is arranged slantwise with the first end 16 lower than the second end 18. An inclined angle θ is defined between the casing 10 and the horizon.

Then slurry 40 is filled into the casing 10. A feeder 50 is applied for filling the slurry 40 into the casing 10. The feeder 50 has an opening 52 for the slurry 40 to flow therethrough. The feeder 50 is arranged adjacent to the second end 18 of the casing 10 with the opening 52 extending into the hole 180 of the second end 18. The slurry 40 is obtained by mixing the necessary powders, for example, metal powders, with a solvent, a binder and, if desirable, some other additives. These components are mixed together in a certain proportion either by weight or by volume. The solvent, which is used to lower the viscosity of the slurry 40 so that the slurry 40 can flow more easily, may be selected from organic material such as ethanol, xylene or the like, which is sensitive to temperature. The binder is used to bind the powders together, and may be selected from polyvinyl alcohol (PVA), polyvinyl butyral (PVB), acrylic resin or the like. Other additives that are desirable may include a dispersant to stabilize the powder against colloidal forces and a plasticizer to modify the properties of the binder. The dispersant may be selected from fish oil such as menhaden fish oil, and the plasticizer may be selected from butyl benzyl phthalate or polyethylene glycol.

During the filling process, the slurry 40 flows from the feeder 50 via the opening 52 into the casing 10. The amount of slurry 40 filled into the casing 10 is determined by the volume of the casing 10 and the inclined angle θ. It is easy to be understood that the maximum amount of slurry 40 filled into the casing 10 is to keep the slurry 40 from flowing out through the hole 160 of the first end 16 of the casing 10. For the casing 10 arranged slantwise, the slurry 40 in the casing 10 has an unevenly thickness along the axial direction of the casing 10 after the filling process. The thickness of the slurry 40 gradually decreases from the first end 16 to the second end 18 of the casing 10. The slurry 40 adjacent to the first end 16 has the largest thickness, whilst the slurry 40 adjacent to the second end 18 has the smallest thickness.

Then the casing 10 filled with the slurry 40 is rotated. The machine 30 is started to drive the casing 10 into rotation along a central axis X-X thereof. The slurry 40 abuts an inner wall of the casing 10 intimately by the centrifugal force during rotation. The slurry 40 is approximately evenly adhered to the inner wall of the casing 10 along a circumferential direction thereof after a period of time of the rotation (As shown is FIG. 4).

During the rotating process, a heating device (not shown) which is arranged under the casing 10 heats the slurry 40 with a relatively low temperature to remove the solvent from the slurry 40 and thus to dry the slurry 40 to form a green layer 60. As the solvent is sensitive to temperature, the solvent turns into vapor by the heating of the heating device. An airflow generated by a fan flows through the casing 10 for facilitating the dissipation of the solvent vapor from the casing 10 to the ambient environment through the holes 160, 180 thereof. Since only a relatively lower temperature is needed, the binder contained in the slurry 40 is not removed. The binder binds the powders together after the solvent is removed from the slurry 40. Thus the green layer 60 having a thickness gradually decreasing along the axial direction from the first end 16 to the second end 18 of the casing 10 is formed by centrifugation formation technology (As shown is FIG. 5).

The casing 10 with the green layer 60 is then sintered under a high temperature to thereby produce the sintered powder wick 200 of the heat pipe as shown in FIG. 1. As with the uneven thickness of the green layer 60, the wick structure 200 formed by sintering the green layer 60 has an uneven thickness, which gradually decreases from the first end 16 to the second end 18 of the casing 10. Finally, the casing 10 is vacuumed and a working fluid such as water, alcohol, methanol, or the like, is injected into the casing 10 via the open ends 16, 18, and then the open ends 16, 18 of the casing 10 is hermetically sealed to form the heat pipe of FIG. 1.

The advantage of the procedure in relation to other methods, e.g. the conventional sintering process, is that the method involves application of the centrifugation formation technology, which can avoid using a core rod (i.e., mandrel) in manufacturing the wick structure 200. Further since the green layer 60 is formed with an uneven thickness along the axial direction of the casing 10, the wick structure 200 is also formed with an uneven thickness. When the heat pipe is applied to absorb heat from the heat generating component, the portion of the heat pipe adjacent to the first end 16 is the evaporating section 120 which thermally attaches to the heat-generating device to absorb heat therefrom, and the portion of the heat pipe adjacent to the second end 18 is the condensing section 140 which thermally attaches to a radiator to dissipate the heat.

FIGS. 6-8 show another example of the method to manufacture the wick structure 200 for the heat pipe. Except for the casing 610 applied for making the heat pipe, other components and the procedure of the second embodiment of the method are substantially the same with the previous embodiment. In this embodiment, the casing 610 has first and second ends 616, 618 at two opposite ends thereof. The second end 618 is an open end and defines a hole 619 therein. The first end 616 of the casing 10 is closed. The casing 610 is set on the machine 30 slantwise with the sealed first end 616 at a lower position. Thus, during the filling process the slurry 40 filled into the casing 610 by the feeder 50 is unable to flow through the first end 616 of the casing 610. The maximum amount of slurry 40 which can be filled into the casing 610 is increased, in comparison with the first embodiment. On the other hand, a relatively larger inclined angle θ can be formed between the casing 610 and the horizon. The difference of the thickness of the wick structure 200 formed by this embodiment between the evaporating section 120 and the condensing section 140 can be increased. Thus a further larger capillary force can be developed by the wick structure 200 corresponding to the evaporating section 120, and a further smaller thermal resistance can be obtained by the wick structure 200 corresponding to the condensing section 140 of the heat pipe.

It is understood that the invention may be embodied in other forms without departing from the spirit thereof. Thus, the present example and embodiment are to be considered in all respects as illustrative and not restrictive, and the invention is not to be limited to the details given herein. 

1. A heat pipe comprising: a casing receiving working fluid therein, the casing comprising an evaporating section for thermally attaching to a heat-generating device to absorb heat therefrom, and a condensing section for thermally attaching to a radiator to dissipate the heat; and a wick structure formed at an inner wall of the casing, the wick structure at the evaporating section having a thickness larger than that of the wick structure at the condensing section.
 2. The heat pipe of claim 1, wherein the thickness of the wick structure gradually decreases from the evaporating section to the condensing section.
 3. The heat pipe of claim 1, wherein the wick structure is sintered powder wick structure.
 4. A method for manufacturing a sintered powder wick structure of a heat pipe comprising the following steps: providing a hollow casing arranged aslant to horizon; filling a slurry of powders necessary to construct the wick into the casing; rotating the casing with the slurry to form a slurry layer held against an inner wall of the casing; sintering the slurry layer in the casing to form the wick structure.
 5. The method of claim 4, wherein the casing is arranged on a centrifugal forming machine which is applied for driving the casing into rotation, a surface of the centrifugal forming machine supporting the casing and the horizon define an inclined angle therebetween.
 6. The method of claim 4, wherein the slurry is obtained by mixing the powders with solvent, binder and additives.
 7. The method of claim 6, wherein during the rotating process the solvent of slurry layer is removed.
 8. The method of claim 7, wherein a heating device is applied for heating the slurry layer to remove the solvent.
 9. The method of claim 7, wherein a fan is applied for generating an airflow to facilitate the removal of the solvent.
 10. The method of claim 4, wherein the casing has an open end and a closed end, the closed end being located lower than the open end.
 11. The method of claim 4, wherein the casing has two open ends.
 12. A method for forming a heat pipe comprising: setting a pipe-shaped casing slantwise, the casing having first and second ends with the second end located higher than the first end and being opened; filling slurry comprising metal powder, solvent and binder into the casing from the second end and rotating the casing; heating the slurry at a first temperature to remove the solvent to obtain a green layer attached to an inner wall of the casing wherein a thickness of the green layer gradually increases from the second end toward the first end; heating the green layer at a second temperature higher than the first temperature to sinter the metal powder to obtain a wick structure in the casing, the wick structure having a thickness gradually increasing from the second end toward the first end; injecting working fluid into the casing, vacuuming the casing and sealing the casing to obtain the heat pipe.
 13. The method of claim 12, wherein the first end is opened with a diameter smaller than that of a middle portion of the casing.
 14. The method of claim 12, wherein the first end is closed.
 15. The method of claim 14, wherein a portion of the obtained heat pipe corresponding to the first end of the casing is used for absorbing heat, and a portion of the obtained heat pipe corresponding to the second end of the casing is used for releasing heat.
 16. The method of claim 12, wherein a portion of the obtained heat pipe corresponding to the first end of the casing is used for absorbing heat, and a portion of the obtained heat pipe corresponding to the second end of the casing is used for releasing heat. 